This invention relates to a wiring structure of a substrate and, in particular, to a wiring structure of a substrate, which is capable of assuring a transmission quality of a high-frequency (high-speed) signal.
In a wiring board for transmitting a high-frequency signal, various proposals for improvement in characteristic impedance and EMI (Electro-Magnetic Interference) protection have been made in order to secure a transmission quality of the high-frequency signal (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2002-344149).
Referring to FIG. 1, an organic substrate adapted to mount an integrated circuit is a built-up substrate having a multi-layer structure. The organic substrate comprises a plurality of first signal wires 501 and a plurality of second signal wires 502. Between the first and the second signal wires 501 and 502, a first insulating layer 503, a first power supply layer (GND layer) 504, a second insulating layer 505, a second power supply layer (Vcc layer) 506, and a third insulating layer 507 are successively laminated.
In the built-up substrate illustrated in FIG. 1, the signal wires are arranged in parallel to the power supply layer via the insulating layer. With this structure, the signal wire is assured to have a continuous characteristic impedance so that a transmission quality is secured when a high-frequency signal is transmitted.
In the above-mentioned built-up substrate, a sink current, a source current, and a through current required upon switching a transistor present in a semiconductor circuit can be secured by a parasitic capacitance formed by the first and the second power supply layers 504 and 505 and the second insulating layer 505 formed between the first and the second power supply layers 504 and 505 or by an external bypass capacitor.
In the above-mentioned built-up substrate, a return current of a high-frequency current on the signal wire 501 flows to the second power supply layer 506 while a return current of a high-frequency current on the signal wire 502 flows to the first power supply layer 504. Therefore, it is possible to secure a return path of the return current.
However, a semiconductor substrate reduced in size and profile and a single-layer or a double-layer organic substrate can not have a multi-layer structure and are therefore disadvantageous as will be described hereinafter.
Referring to FIG. 2, a single-layer organic substrate has a plurality of first signal wires 601, an insulating layer 602, and a plurality of second signal wires 603. In case of such a substrate which can not have a multi-layer structure, there are following problems:
(1) It is difficult to secure continuity of a characteristic impedance of the signal wire.
(2) It is difficult to secure a return path of a return current.
(3) It is difficult to achieve a low impedance of a power supply line.
Thus, it is difficult to secure a transmission quality of a high-frequency signal.